There are several semiconductor photodetectors sensitive to wavelengths of 1.0 to 1.6 .mu.m for optical telecommunication systems, such as a PIN photodetector including a light absorbing layer of In.sub.0.53 Ga.sub.0.47 As lattice matched to an InP substrate as described on pages 653 and 654 of "Electronics Letters, vol. 20, 1984", or an avalanche photodetector as described on pages 257 and 258 of "IEEE. Electron Device Letters, vol. 7, 1986". The photodetector such as an avalanche photodiode has been used in long distance optical telecommunications systems, because it has an advantage in inner gain effects and high speed response due to avalanche multiplication.
One type of a conventional avalanche photodiode includes a buffer layer formed on a substrate, an avalanche multiplication layer formed on the buffer layer, a light absorbing layer formed on the avalanche multiplication layer, and a photodetecting region formed on the avalanche multiplication layer.
In operation, light is supplied to the avalanche photodiode which is applied with a reverse bias voltage. The light is absorbed at the light absorbing layer to generate photocarriers that are electrons and holes. The photocarriers are injected into the avalanche multiplication layer to cause ionization impacts which results multiplication under an intensive electric field. It is desirable that the ionization impacts in the avalanche multiplication layer are carried out only by the photocarriers injected from the light absorbing layer. Therefore, it is desirable that the electron and hole ionization rates .alpha. and .beta. are vastly different (.alpha.&gt;.beta. or .alpha.&lt;.beta.) and the photocarriers injected from the light absorbing layer initiate the avalanche process to provide an avalanche photodiode having low-noise and high speed response characteristics. The ratio .alpha./.beta. depends on property of material of which the avalanche multiplication layer consists. In an InGaAs type avalanche photodiode having an InP avalanche multiplication layer in which holes are injected carriers, for instance, the ratio .beta./.alpha. of InP is up to 2 at the most, which is far smaller than the ratio .alpha./.beta. of Si which is approximately 20.
Capasso et al have suggested that the ratio .alpha./.beta. can be controlled artificially by using a superlattice structure having large band energy discontinuity of the conduction band (.DELTA.E.sub.c) as an avalanche multiplication layer, as described on pahes 38 to 40 of "Applied Physics Letters, vol. 40, 1982".
According to the conventional avalanche photodiode including the superlattice structure, however, there is a disadvantages in that noise and speed response characteristics are not sufficient, because the ratio of ionization rates which is dependent on material of which the avalanche multiplication layer of the avalanche photodiode consists is not sufficiently high. Further, there occurs a so-called pile-up of holes at the band energy discontinuity of the valence band (.DELTA.E.sub.v), so that the band width may be reduced. In order to prevent the pile-up of holes, it is proposed that the avalanche multiplication layer should consist of InAlAs/InGaAsP or AlGaAsSb/AlGaInAs structure so that the band energy discontinuity of the valence band .DELTA.E.sub.v becomes zero. However, the band energy discontinuity of the conduction band .DELTA.E.sub.c is also reduced, so that the ratio of the ionization rates is reduced.